Conventionally, fastening methods for screwing a body to be fastened such as an apparatus by predetermined fastening force using fastening screws such as bolts may include, for example, a torque fastening method using a torque wrench and a (plastic) a rotation angle method in which the amount of tightening is specified by the angle of tightening.
For tightening a screw by the plastic rotation angle method, the screw is first tightened at a specified torque value (snug torque or torque at the yield point) as tightening in the elastic region of the screw. After that, the wrench is subsequently turned by a predetermined angle as tightening in the plastic region of the screw in order to further tighten the screw.
Tightening tools to be used for tightening screws by the rotation angle method may include an angle wrench (see Patent Literature 1). The angle wrench in Patent Literature 1 has an angular velocity sensor secured to a head member and is configured to detect the operation of a toggle by detecting the displacement of the angular velocity sensor. Then, the rotation angle from the toggle actuation onward is determined from the angular velocity that is measured by the angular velocity sensor.